This report summarizes a program conducted to design and evaluate TCAS II avionics, focusing on the air-to-air surveillance subsystem. The Traffic Alert and Collision Avoidance System (TCAS) is a system of airborne equipment being developed by the FAA for the purpose of preventing mid-air collisions. TCAS is intended as a collision avoidance backup to the existing system of air traffic control. In one mode of operation, TCAS would prevent a collision between two aircraft, each equipped with a unit called TCAS II. Each TCAS II would sense the presence of the other aircraft, measure its location (in range, altitude, and bearing), detect a hazardous situation if one develops, and then display a resolution advisory (such as climb or descend) to the pilot, after first carrying out an automatic coordination between the two aircraft to assure that the action taken by one aircraft will complement the action taken by the other aircraft. The TCAS II also affords protection against aircraft equipped with either Mode S or existing Secondary Surveillance Radar transponders. The TCAS II also affords protection against aircraft equipped with TCAS I which is a simpler form of TCAS.